Reference oscillators are used as precise and stable frequency references in innumerable applications. They are used for obtaining a precise frequency reference and/or for obtaining a precise time reference. Such reference oscillators are needed, e.g., in devices which communicate using radiofrequency links, or otherwise use precisely defined frequencies. Reference oscillators are also needed in precise clocks and timing circuits.
The critical requirements for a reference oscillator are (1) high long-term stability (low drift/ageing), (2) low phase noise, (3) high thermal stability, e.g., low thermal coefficient, and (4) precise value for the reference oscillator frequency. In most high-volume applications, the following features are also important: (5) small size, (6) low power consumption, (7) high integration level between the resonator, the oscillator electronics, and the device package, and (8) low cost.
Quartz crystals are typically used for resonators in high-quality reference oscillators. Disadvantages of these crystals are their large size and incompatibility for monolithic integration with electronics.
Modern micromachining makes it possible to fabricate miniaturized mechanical resonators (Micro Electro Mechanical Systems=MEMS) with resonance frequencies ranging from several kHz up to GHz range. Examples of such microresonators based on surface or bulk micromachining of silicon are presented in H. J. De Los Santos, “RF MEMS Circuit Design for Wireless Communications”, Artech House, Boston/London, 2002. The advantages of microresonators include small size, low power consumption, and possibility for increased integration level between the resonator, the oscillator electronics, and the device package. Both monolithic integration and the system-on-chip approach are viable solutions for increasing the integration level of a reference oscillator. Monolithic integration of micromachined resonators and integrated circuits will facilitate also more complicated micro-electro-mechanical circuits.
There exist, however, several complications related to realizing a MEMS-based reference oscillator. (i) A reference-quality long-term stability is challenging to achieve using microsize resonators, in particular when a good phase noise performance is to be obtained from the same device. (ii) The thermal coefficient for the resonance frequency of the microresonators is typically in the range [−10, −40] ppm/K and are thus far too large for typical reference oscillator applications. (Proposed solutions to temperature dependence compensation include adjusting the bias voltage, see for example U.S. Pat. No. 5,640,133, MacDonald et al, adjusting the width of the capacitive coupling gap, see for example U.S. Patent Application 20030051550, Nguyen and Hsu, or utilizing mechanical stress, see for example U.S. Patent Application 20020069701, Hsu and Nguyen.) (iii) The small dimensions in microsize resonators make considerable challenges to fabrication tolerances to obtain precise resonator frequencies. In summary, demonstrating a MEMS reference oscillator having simultaneously all the desired properties (1)-(4), has turned out to be difficult.